Olefins are the basic industrial synthetic building blocks for producing common plastics: synthetic fibers, glycols, and various surfactants. Ethylene and propylene are typically produced via steam cracking of naphtha, ethane, and other hydrocarbon feedstock, followed by distillation. In the conventional distillation process, ethylene/ethane, or propylene/propane mixtures are liquefied for cryo-distillation in large fractionating columns, which involves phase change and consumes a huge amount of energy. To reduce the energy consumption, alternative technologies have been actively sought after in both industry and academic research community. As a result, a number of processes have been proposed to solve the problem. Adsorption appears to be an attractive alternative because of the maturity of the basic technology. However, to obtain the olefin products with desirable purity, pressure-, vacuum-, and temperature-swing adsorption (PSA, VSA, and TSA) processes have to be operated in at least a four-bed, five-step fashion that results in a high energy demand and a large capital investment.
Membrane separation for olefin/paraffin mixtures offers an appealing energy-efficient alternative to the cryo-distillation process. Great effort has been dedicated in developing a high flux, high selectivity olefin separation membrane. The state-of-the-art membrane cannot meet the stringent requirements of real life olefin/paraffin separation, mainly due to problems such as trade-off between selectivity and flux; poor stability, especially in practical operating environments; and fouling or poisoning in the presence of contaminants (Olefin, alkynes, dienes, olefin sulfide, and other sulfur species).
Membrane candidates have been proposed ranging from polymers, ceramics, to the composites of polymers and ceramics. Although polymeric membranes have been used successfully in several gas separation applications, including nitrogen production from air and olefin removal from refinery streams, the selectivity and gas fluxes of such membranes are inadequate for separating olefins from saturated hydrocarbons. Under industrial operating conditions, those polymeric membranes suffer from plasticization. Even the best polymeric membranes can only offer olefin/paraffin selectivity of 4-5. To replace or supplement for the separation of olefin/paraffin in steam crackers or propane de-olefination plants, membranes with selectivity of twenty or more are needed.
Facilitated transport membranes have attracted research interest for a long time. Facilitated transport membranes incorporate a reactive carrier in the membrane, which reacts with and helps transport one of the components of the feed across the membrane. High loadings (more than 80 wt % or 40 vol %) of silver salt (AgBF4) as the carrier were normally used in the polymer membranes. Mixed-gas ethylene/ethane selectivity of more than 50 has been reported (U.S. Pat. Nos. 6,414,202, 6,525,236, and 7,479,227). The main hurdles, however, include the lack of carrier stability due to washout of silver ions and the need for water vapor in the feedstock. While complexing with olefins, the carriers also tend to react with other species causing undesirable carrier deactivation or poisoning over short time.
Inorganic membranes, such as carbon membranes (A. F. Ismail, L. I. B. David, J. Membrane Sci., 2001, 193, 1-18), and zeolite membranes have also been investigated for the separation of olefin/paraffin gas mixtures. Inorganic membranes have much greater thermal and chemical stability than polymer membranes. A few studies have indicated that zeolite membranes, specifically FAU (pore size 0.74 nm), and ETS-10 (pore size ˜0.56 nm) zeolite membrane, can separate olefins from paraffins. It has been reported that FAU-type zeolite membranes synthesized by secondary growth method can reach a separation factor for propylene/propane mixtures of 13.7±1 at 100° C., with the corresponding propylene permeance of 0.75×10−8 mol/m2·s·Pa, which surpassed the performance of polymer membranes as well as carbon membranes (I. G. Giannakopoulos, V. Nikolakis, Ind. Eng. Chem. Res., 2005, 44, 226-230). Tiscornia et al, reported a method to prepare an ETS-10 zeolite membrane and its application in propylene/propane separation (I. Tiscornia, S. Irusta, C. Tellez, J. Coronas, J. Santamatia, Journal of Membrane Science, 2008, 311, 326-335). The membrane can only achieve propylene/propane selectivity of about 3-5. Hence, the separation factors of these pristine membranes are not high enough for practical olefin and paraffin separation in the industry.
ETS-10 zeolite with mixed a coordination metallosilicate framework and general formula Na2TiSi5O13 was first discovered by Kuznicki et al. (U.S. Pat. No. 4,853,202 and U.S. Pat. No. 6,517,611). The modified. ETS-10 zeolite powders were reported to be a good candidate to selectively adsorb ethylene over ethane (U.S. Pat. No. 8,017,825). The powder cannot be used for continuous separation of olefin from paraffin.